two pumps in series

I have a wood stove with a wet back. I'm using a Grundfos UPS 25-40 pump to circulate the water. Some of the radiators are not getting hot, particularly the ones far down the line. I had a heating guy look at it and he straight away said that my pump wasn't powerful enough and recommended I upgrade the pump.

The pump is on the hot side, it pulls hot water from the stove and pushes it around the house. All the radiators are in parallel.

I have an identical spare pump lying around. Can I just put this pump in series on the cold side (the input to the wet back) and have it push water through the stove as well? Would this not work like two locomotives and essentially double the pumping power?

There's a second reason I want to do this. Sometimes when air gets caught in the boiler (the wet back), it starts to boil and if enough air is in there, I get steam pushing all the way into the pump and the pump goes into vapor lock--it can't pump vapor, it's designed to pump liquid. The system goes into high pressure, the safety valves start blowing steam and the whole system needs to be cooled down before it explodes!

If I have a second pump on the cold side, this pump would never get steam and hence I should never get into a state where the circulation could stop. Hence, I'm viewing it as a safety measure.

The question is, is there anything wrong with putting in a second pump in series like this? Or should I just get a larger pump and not use the identical second one I have? Is the existing pump on output (hot) side in the correct place? Or should the circulator pump be on the input (cold/return) side?

If it is ok to use two pumps in series like this, any reason why one on the hot side and one on the cold side might cause a problem? Hot water being less dense than cold water and the stove heating the water means there's more volume going out than in. Is this going to create a situation where one pump is always ahead or behind the other and create some sort of imbalance?

Because of the differing lengths of pipe to the various radiators and possibly their size, if they differ, it sounds like you're not getting the same flow through all of them simply because the water will take the path of least resistance...those further away, or with more restrictions, elbows, etc., just won't get the flow. The way around this is to use balancing valves so that they all 'look' the same, and the flow will then equalize. Increasing flow probably won't help, balancing should.

So, this lovely chart shows that water at at 90C is about 3% more volume than water at 40C.

I don't know if that's "in the noise" for a pump. If the two pumps are so close in head pressure then the cold side pump is always going to be pushing more water through the hot pump (it pushes the same amount of water but then it expands 3% in the boiler). Is this a problem?

Mgrant, the pump you are using is not a positive displacement type. The difference in volume will not be a problem.
This type will pump water through a second pump even if it is powered off.
Your idea of adding one on the cooler input will work fine & pressurize the existing one on the hot output side, elemonating possible cavitation.
They would be in series, the heads will add together for about 1/3 more flow.
To double the flow, the head has to be about 3.5 x

Regardless of how much flow you have, that flow WILL take the path of least resistance! You'll still need to balance the branches to get even heat to all of the radiators.

Look up how many gpm flow the existing pump should have in your system. That would help determine if it can move enough water at a certain temperature to distribute the heat you need. If it can, adding another pump won't balance the system and get heat everywhere. Too high of a volume just ends up costing more and creating water flow noises and potentially eroding the pipe.

You need to address the air problem 1st and then figure out why you have circulation problems. As has been said water goes to the path of least resistance and balancing is probably called for but regardless that grundfos pump is plenty large enough to handle the flow.

BadgerBoilerMN, I'm not sure what you mean by phase change and control problem. For me, the control of the second pump is simple, it's just wired in paralleled with the first.

Tom Sawyer, I understand the air problem pretty well at this point. There is a high pressure release valve behind the stove. I have replaced it twice now. For some reason, it just doesn't seem to let all the air out when filling the system. I'm going to put a second very simple screw type valve near it so I can manually get most of the rest of the air out. The reason there is air in there in the first place is that this just is a natural high point in the system. The pipe dips down coming out of the stove (a few cm / a couple of inches), so it's just really hard to get all the air out of there when the system is full, even through the security valve. Hence, adding a little valve there should allow me to do that.

I totally agree on the need to make sure the system is balanced. Two people have told me that the few long runs I have, the balancing act may not be enough to force water down those long runs due to friction and hence a second pump should help that.